1. Field of the Invention
The invention is related to a method of bonding composite structures together by means of treating the composite surfaces with an atmospheric plasma, followed by applying adhesive to one of the surfaces, joining the composites together, and curing said adhesive.
2. Description of the Related Art
Composite materials are used in the aerospace, automotive, electronics, medical and sports equipment industries because of their unique properties (M. M. Schwartz, “Post Processing of Composites,” Society for the Advancement of Materials and Process Engineering, Covina, Calif. 1996). They are lightweight, exceptionally strong, chemically and thermally stable, and can be processed into a wide variety of shapes. Their high strength to weight ratio makes them especially attractive for use in aircraft, where by replacing the metal structures, they can reduce the weight of the vehicle and dramatically save on fuel costs.
A composite is composed of a reinforcement and a matrix material (M. M. Schwartz, ibid.). The reinforcement may be fibers, whiskers, platelets, flakes, and other shapes. Fibers are the most common, and among these glass, graphite and aramid fibers are widely used. Matrix materials may be thermosetting or thermoplastic polymers, metals, or ceramics. Polymers are attractive because they are strong and lightweight, thermally and chemically stable, and are conveniently processed into desired structures. Thermosetting resins react at elevated temperature to produce three dimensional crosslinked networks. Once reacted, thermosets remain fixed in shape, and cannot be reprocessed. Thermosets comprise polyesters, epoxy resins, phenolic resins, bismaleimides and polyimides. Thermoplastics are polymers that do not chemically react when heated. Instead they may be melted and formed into engineered structures, and can be reprocessed if desired. They offer the potential of being tougher and more stable than thermosets. Examples of thermoplastic matrix materials include, but are not limited to, polyethersulfone (PES), polyetherimide (PEI), polyether-etherketone (PEEK), and polyetherketoneketone (PEKK).
Carbon-fiber-reinforced PEEK is an especially attractive composite. PEEK is an aromatic polymer that has excellent mechanical strength, and high resistance to fatigue, impact and abrasion. It is also fire resistant and a good electrical insulator (J. Jang and H. Kim, Polymer Composites, Vol. 18, p. 125 (1997); and M. M. Schwartz, ibid.). In the medical industry, PEEK is an attractive material for implants due to its biocompatibility. In the aerospace and automotive industries, metal structures are being replaced by carbon-fiber-reinforced PEEK, because of its superior mechanical properties and high strength-to-weight ratio.
Although carbon-fiber-reinforced composites have attractive properties for use in many engineered products, they suffer from a major drawback in that these materials often do not adhere well to other surfaces. Without specialized surface treatments, composites may not form strong bonds to epoxy resins (Schwartz, ibid.). This is especially true of thermoplastics, such as PES, PEI, PEEK and PEKK. Adhesives and surface preparation methods designed for other composites do not provide sufficiently strong bonds, so that under a relatively low shearing force they undergo adhesive failure with the glue separating cleanly from the polymer surface. Therefore to use these materials in aircraft and other structures, the thermoplastic composites must be bolted together. This approach produces a weaker structure, and increases the total weight of the aircraft.
Many methods have been examined for treating polymer composites prior to adhesive bonding. These include bead blasting, wet chemical etching, flame oxidation, UV/ozone treatment, argon ion bombardment, and oxygen plasma etching (M. M. Schwartz, ibid.). Among these methods, oxygen plasma treatment has been found to be the most effective at increasing the bond strength of epoxy adhesives to thermosetting and thermoplastic composites. N. Inagaki and coworkers, (“Surface modification of poly (aryl ether ether ketone) film by remote oxygen plasma,” Journal of Applied Polymer Science, Vol. 68, p. 271 (1998)) describe the treatment of PEEK with a low-pressure oxygen plasma operated at 0.13 Pascal. After exposing the PEEK film for 30 s, the water contact angle dropped from 93 to about 60 degrees. J. Comyn, et al. (“Corona discharge treatment of polyetheretherketone (PEEK) for adhesive bonding,” Intl. Journal of Adhesion and Adhesives, Vol. 16, p. 97 (1996)) subjected PEEK film to an oxygen plasma for 60 s at 40 Pascal. These authors observed a decrease in the water contact angle. After bonding samples together with epoxy adhesive, they found that the plasma treatment yielded a large increase in peel strength and a doubling of the lap shear strength over untreated samples.
One of the disadvantages of oxygen plasmas used in the prior art is that they were generated in a vacuum (M. M. Schwartz, ibid.). To receive treatment the composite parts must be inserted into a sealed chamber and the gas pumped away prior to striking the discharge. This approach limits the size and shape of parts that can be treated, since they must fit inside the chamber. Moreover, vacuum operation requires expensive equipment that must be maintained, is more time consuming, and more expensive than atmospheric pressure processes.
The use of atmospheric pressure plasmas to treat polymer composites has been described in the prior art. In particular, coronas and dielectric barrier discharges have been used to treat polymers for increased wettability and surface adhesion (see for example, P. J. Ricatto, et al., “Chemical processing using non-thermal discharge plasma,” U.S. Pat. No. 6,923,890, Aug. 2, 2005; L. A. Rosenthal and D. A Davis, “Electrical Characterization of a Corona Discharge for Surface Treatment,” IEEE Transaction on Industry Applications, Vol. 1A-11, p. 328 (1975); and J. Comyn, et al., “Corona-discharge treatment of polyether-etherketone (PEEK) for adhesive bonding,” Intl. Journal of Adhesion and Adhesives, Vol. 16, p. 301 (1996)). The latter authors showed an approximate doubling of the lap shear strength following corona treatment, similar to what was achieved with the vacuum oxygen plasma. Nevertheless, the dielectric barrier discharge and the corona generate the atmospheric pressure plasma between two closely spaced electrodes separated by a dielectric spacer. In order to treat the composite, it must be in the form of a flexible film less than 1.0 mm thick, so that it can be fed between the two electrodes. This design severely restricts the type of composite materials that can be treated, and is not applicable to composites that have been formed into rigid three-dimensional shapes, such as those used in the structure of an aircraft.
Therefore, there is a need for a method of adhesively bonding a composite work piece of any size and shape that utilizes an effective surface treatment of the material, so that when the part is joined with another work piece, they form a strong, permanent bond. In particular, there is a need for a method of adhesively bonding a composite work piece where an atmospheric pressure plasma is used for surface treatment, such that the plasma can be conveniently applied to the surface of any composite structure, regardless of its size and shape, and after treatment and application of the adhesive a strong, permanent bond is obtained. As described hereafter, these and other needs are met by embodiments of the present invention.